It is well established in the manufacturing of lime/cement products that limestone which is initially quarried exposed to additional unit operations for processing is eventually calcined with various kilns, examples of which are the vertical or rotary type kilns. In these kilns, the limestone is calcined at a high temperature, generally 1200° C. or greater at which point the material is converted to lime. In the calcination process carbon dioxide is removed from the calcium carbonate to leave calcium oxide. The lime may be hydrated by treatment with water to create calcium hydroxide which generally is in the form of a fine white powder which can be pulverized to any size distribution depending upon the final use of the product.
In view of the fact that the kilns are used to generate high temperatures for the conversion process, result is the generation of a flue gas at the lime/cement plant. The gas may contain some unwanted pollutants, which may become a problem in terms of pollution emission. This may aim as an issue in light of increasingly stringent environmental requirements to reduce air pollution significantly. Numerous advances have been made in pollution control for this industry. Currently, there are wet, semi-wet and semi-dry scrubbing systems which require the use of an external source of sorbent. The sorbent most desirable is the hydrated lime for flue gas scrubbing.
Gebhardt, in U.S. Pat. No. 6,331,207, issued Dec. 18, 2001, teaches a method of treating cement kiln dust for recovery and recycle. In the patent, it is indicated that the supply of cement kiln dust may be moistened and subsequently treated with carbon dioxide to convert the materials to carbonates. During the carbonation cycle, the water in the hydroxides is released to formulate slurry. The soluble alkalis and sulfate are released in the liquid phase with the solids being separated from the liquid. The solids, subsequent to washing, provide a useful feed to the kiln. The liquid contains alkali salts. The gases from the kiln are conditioned by condensation of water and the removal of ammonium compounds.
A further wet process is taught in U.S. Pat. No. 4,716,027, issued Dec. 29, 1987, to Morrison. The reference relates to a simultaneous scrubbing and removal process where acidic exhaust gases are scrubbed and metal salts are removed from alkaline waste dust to produce useful products. Similar to the above-mentioned reference, this reference relies on contact of the gas to be scrubbed with basic slurry.
Martin et al., in U.S. Pat. No. 4,195,062, issued Mar. 25, 1980 discloses a further variation on the wet scrubbing process where a portion of the effluent in a limestone flue gas scrubbing system is removed to a reaction tank which has a controlled pH. A slurry material is re-circulated into a spray tower for contact with the flue gas.
In view of the prior art techniques that have been employed, it would be desirable to have a process that is either semi-wet or dry which is self sustaining and does not require the addition of materials to formulate a sorbent for flue gas scrubbing. The present invention is directed to satisfying this need.